After a long, hot and dry summer that left many parts of the country in drought, rain is finally forecast this week — and in some places it is set to be heavy. Many people rely on phone apps to decide whether to carry an umbrella or sunscreen, but one aspect of those apps often causes confusion: what does a 70% chance of rain actually mean?
How forecasters calculate the percentage
Grahame Madge, a spokesperson for the Met Office, explained that the figure comes from complex calculations. Meteorologists gather weather data from around the world — from ocean buoys to satellites — and compile it into what is known as an ensemble forecast.
“Ensemble forecasting, basically it's like running several weather forecasts with slightly different starting positions, and if 70% of those forecasts show rain, then that is where the 70% chance of rain will come from,” he said.
“Some people think it's by area or some other metric, but actually if you run a forecast 10 times, and seven of those runs show rain, then that is where the 70% rainfall comes from.”
Why forecasts never reach 100%
There is no way, yet, for a forecast to be totally accurate. Madge noted: “You'll notice on the app that we only go down to around 5% or up to 95% because calling 100% in meteorology is not technically possible, because you can always get something that's just outside of the forecast.”
Long-time weather reporter and former meteorologist Crystal Egger agreed. “If the forecast says there’s a 60% chance of rain, it means there’s a 60% probability that your specific location will receive measurable rain during that period. It doesn’t mean it will rain for 60% of the day or that 60% of the area will get rain,” she said.
She suggested a simple way to visualise the probability: “One way I like to visualize probability is with a group of 10 people. If all 10 are in the same predefined area and there’s a 50% chance of seeing rain, you could visualize that as five out of 10 people seeing rain. With a 60% chance, it would be six out of 10.”
She also stressed that it is not an exact science. “You can have an 80% chance of rain and only get a few hundredths of an inch, or a 20% chance and get a significant downpour if a storm happens to move over you.”
How to use the forecast practically
Madge said he would like the public to understand more about how meteorologists turn complex data into the little emoji on the app. He advised: “We're not expecting people to become meteorologists overnight, but if the next day's symbol for the weather shows rain and you want the day out, just look below that and look at the hourly rainfall, because that will give you a good idea if there's a good usable part of the day.”
“If it's only light rain, does that stop you? Could you be weather ready and take an umbrella or raincoat for the children and wellies?” He added: “Let's not let the weather be a block just because you see a rain symbol, it doesn't mean to say that you're gonna get rain at the particular time that you want to go out.”
